This invention relates generally to a power transmission belt for a continuously variable transmission (“CVT”), more particularly to a CVT belt with a fiber-loaded rubber composition, and specifically a rubber composition based on polyolefin elastomer with both pulp and staple high-modulus fiber.
A CVT generally has some kind of closed-loop control or feedback mechanism for automatic and relatively rapid shifting based on the dynamics of the drive in a system. Often, in a CVT the driver sheave is controlled based on or reacts to a speed measurement or speed change in order to keep the power source or motor within an optimum power or speed range, and the driven sheave is controlled based on or reacts to the torque load. The variable-pitch sheaves may be adjusted by various mechanisms including mechanical, electro-mechanical, electronic, hydraulic, or the like. Belt-driven CVTs are widely used in scooters, all-terrain vehicles (“ATV”), snowmobiles, agricultural equipment, heavy equipment accessory drives, and other vehicles. Generally, as two pulley halves move axially apart or together to force a change in belt radial position in a CVT, the belt may be subjected to extreme friction forces as the belt changes radial position within the sheaves. As two sheave halves move together axially to increase the pitch line of the belt, the belt is subjected to extreme friction forces and to high axial or transverse compressive forces. High and variable torque loads result in high tension forces and high wedging forces which also result in high transverse compressive forces and frictional forces on the belt. Some applications also use the belt as a clutch, resulting in additional frictional forces on the contact surfaces of the belt. All these forces may be very severe in a CVT because of the dynamics of the applications (e.g. frequent, rapid shifts, with high acceleration loads). As the CVT belt traverses the driver and driven pulleys, it is also subjected to continual bending or flexing. Rubber CVT belts are generally used without lubrication in so-called “dry CVT” applications. Thus, the CVT belt needs to have good longitudinal flexibility, high longitudinal modulus, high abrasion resistance, and high transverse stiffness. The belt must operate across a wide temperature range, for a long time.
Representative of the art is U.S. Pat. No. 6,620,068, which discloses a raw-edge double-cogged V-belt for variable speed drives having curvilinear cogs on the inside and outside, a layer of spirally wrapped cords made of fibers such as polyester, aramid, and/or glass fiber. The belt includes compression and tension layers of rubber containing short fibers aligned laterally for transverse reinforcement. The belt also includes a layer of reinforcing fabric on the inside and/or outside cog surfaces.
Also representative of the art is U.S. Pat. No. 4,708,703, which discloses a CVT belt with aligned upper and lower teeth and grooves, and with longitudinal cords. The teeth are preferably covered at their tops with transverse stiffening elements to deal with the problem of buckling and to increase the torque capability.
U.S. Pat. No. 6,485,386 relates to rigid inserts in a cogged V-belt to increase transverse stiffness. Herein and in the claims, the term “rubber CVT belt” excludes the use of such rigid inserts or stiffening elements, as well as the use of external rigid appendages or clips or blocks.
Yet, CVT belts need high transverse stiffness due to the aspect ratio and the high axial forces in use. Various approaches to increasing stiffness have been tried in the past. The most common approach is to incorporate transversely oriented chopped fibers into the belt body. This approach has limits.
U.S. Pat. No. 7,189,785 relates to a blend of HNBR and EPDM or other ethylene-alpha-olefin elastomer. Extensive data on chopped fiber-loaded elastomers is included. It teaches that too much (more than 20 parts weight per hundred parts of elastomer (“phr”) leads to processing problems, without benefits regarding heat build up.
U.S. Pat. No. 8,672,788 relates to a vulcanized rubber CVT belt in the form of an endless V-belt having a belt body with angled sides, a tensile cord layer of helically spiraled tensile cord embedded in the belt body, an overcord rubber layer, and an undercord rubber layer, wherein the tensile cord is a twisted, single-tow bundle of continuous-filament, carbon fiber. It teaches that the use of 18 k carbon cord increased the transverse stiffness of the CVT belt.
U.S. Pat. No. 5,610,217 relates to a power transmission belt with a main belt body portion incorporating an elastomeric composition with an ethylene-alpha-olefin elastomer reinforced with a filler and a metal salt of an α-β-unsaturated organic acid.
U.S. Pat. No. 6,616,558 relates to at least one of said elastomeric belt body portion and said adhesive rubber member exhibits at least one of a complex modulus measured at 175° C., at 2000.0 cpm and at a strain of 0.09 degrees, of at least 15,000 kPa; and a tensile modulus, measured at 10% elongation, of at least 250 psi (1.724 MPa).
U.S. Pat. No. 6,511,394 relates to an elastomer composition with an elastomer blend of low and high molecular weight ethylene-alpha-olefin polymers.
WIPO Publ. No. WO2010/047029A1 relates to a rubber composition for a flat transmission belt comprising an ethylene-α-olefin elastomer.
WO2015/045255A1 relates to a cogged V-belt composition with mixture of short nylon or PET nano-fibers and chopped para-aramid fibers in EPDM elastomer.
U.S. Pat. No. 6,358,171 to Whitfield discloses use of aramid pulp or staple fibers in toothed belts.
It is not known or suggested to use a blend or combination of aramid pulp fiber and aramid chopped fiber in the main belt-body ethylene-alpha-olefin elastomer composition of a power transmission belt.